Catalyst in the form of grains comprising an acidic porous core surrounded by a uniform outer layer

ABSTRACT

The invention relates to a catalyst that comes in the form of grains, each grain consisting of a core covered by at least one external layer, the core consisting of an acidic porous solid and having a size of between 0.1 micron and 0.4 millimeter, in which the external layer has a uniform thickness with a uniformity criterion C, which is less than 0.30, whereby said uniformity criterion C is defined as being equal to an average, on a number N of catalyst grain samples, of the ratio of the difference between the maximum thickness, Ei max , of the external layer and the minimum thickness, Ei min , of this same layer to the average of these two thicknesses Ei max  and Ei min .

TECHNICAL FIELD

This invention relates to the field of catalysts that come in the formof grains each comprising a core with an acidic porous solid base andsurrounded by at least one uniform external layer.

PRIOR ART

International Patent Application WO 97/33684 describes a method forpreparation of a zeolite-based film on the surface of differentsubstrates.

International Patent Application WO 99/28031 describes a catalyst thatcomprises crystals of a first zeolite and a discontinuous layer ofcrystals of a second zeolite.

The catalysts of the prior art have a certain number of drawbacks. Inparticular, the non-uniformity of the external layer does not make itpossible to obtain an optimum diffusional selectivity.

Non-uniformity of the external layer is defined as a discontinuity orvariation of the thickness of said layer around the core. Diffusionalselectivity is defined as the preferred passage of certain reagents orproducts relative to others. It is all the more satisfactory the fasterthe passage and the more effective the separation of the radicals. Theseparation is all the more effective as the covering of the layer istotal, and the passage is all the faster as the thickness of the layeris fine and therefore uniform.

Moreover, the existing processes generally are not suited for coveringwith a uniform layer a core with a porous solid base whose size is lessthan or equal to 0.4 millimeter. The processes for preparation of theprior art are used on porous substrates of a relatively large size, onthe order of a millimeter, with a slight curvature, facilitating thehomogeneous covering by fine layers of nanocrystals.

SUMMARY OF THE INVENTION

A catalyst that comes in the form of grains each comprising a core withan acidic porous solid base and surrounded by at least one uniformexternal layer as well as a process that makes possible the productionof grains of said catalyst have been found, whereby each grain is formedby a small core, i.e., of a size that is less than or equal to 0.4millimeter and that makes it possible to cover said core with at leastone external layer that has a uniform thickness.

The uniformity of the thickness of the external layer can be evaluatedusing a criterion, so-called uniformity criterion C, which is defined asbeing equal to an average, on a number N of catalyst grain samples, ofthe ratio of the difference between the maximum thickness, Ei_(max), ofthe external layer, and the minimum thickness, Ei_(min), of this samelayer to the average of these two thicknesses Ei_(max) and Ei_(min).

The evaluation of this uniformity criterion can be made by any meansthat is known to one skilled in the art, such as, for example from aphotograph or an analysis of images obtained from characterization byelectronic microscopy.

The process for preparation of the catalyst according to the inventioncomprises:

-   -   a) A stage for preparation of crystals or crystal agglomerates        to form the core of each of the grains, the core consisting of a        porous solid,    -   b) A stage for modification of the core of each grain to impart        an at least partial acidity to said core,    -   c) A stage for adhesion of nuclei distributed uniformly over the        surface of the core of each grain, whereby said nuclei consist        of nanocrystals based on the material used for the external        layer, and    -   d) A stage of growth of said nuclei on the core of each grain.

DETAILED DESCRIPTION OF THE INVENTION

An object of this invention relates to a catalyst that comes in the formof grains, each grain being formed by a core covered with at least oneexternal layer, the core consisting of an acidic porous solid and havinga size of between 0.1 micron and 0.4 millimeter, in which the externallayer has a uniform thickness with a uniformity criterion C that is lessthan 0.30, whereby said uniformity criterion C is defined as being equalto an average, on a number N of catalyst grain samples, of the ratio ofthe difference between the maximum thickness, Ei_(max), of the externallayer and the minimum thickness, Ei_(min), of this same layer to theaverage of these two thicknesses Ei_(max) and Ei_(min).

The average with the base of the uniformity criterion C is generallycarried out from a significant number of samples of catalyst grains. Thenumber N of catalyst grain samples is preferably greater than or equalto 100.

The catalyst according to the invention has an external layer whoseuniformity criterion can be expressed, for example, by the followingexpression:

$C = {\frac{2}{N}*{\sum\limits_{i = 1}^{N}\;\frac{{Ei}_{\max} - {Ei}_{\min}}{{Ei}_{\max} + {Ei}_{\min}}}}$in which:

Ei_(min) represents the minimum thickness of the external layer around agrain i, and

Ei_(max) represents the maximum thickness of this same external layerand N corresponds to the number of samples of grains used to carry outthis statistical characterization of the uniformity.

The evaluation of the uniformity criterion can be made by any means thatis known to one skilled in the art, such as, for example, from aphotograph or an analysis of images obtained from characterization byelectronic microscopy.

In the case where the uniformity criterion is measured by scanningelectronic microscopy, the catalyst grains are dispersed in an Epon- orAraldite-type synthetic resin. After polymerization of the resin, theblocks that contain the catalyst grains were demolded and form acylinder of about 5 mm in diameter. They were cut with an ultramicrotome(LKB 8800 ultrotome III) by first using a glass knife. The resin is cutat its end so as to obtain a small surface area (<0.5 mm²) in the shapeof a trapezoid. After detecting a zone of interest, ultrafine sectionsof about 80 nm were made with a diamond knife. The sections were thendeposited on a substrate for observation with a scanning electronicmicroscope. In order to observe the sections, it is necessary to platethe sections with gold by vacuum evaporation (argon atmosphere). Theequipment that is used is a PHILIPS XL-30 microscope with a 10 to50,000× magnification.

By using the criterion C presented in the preceding paragraph, thecatalyst according to the invention has a uniform external layer with auniformity criterion of less than 0.3, preferably less than 0.2, andmore preferably less than 0.1.

Preferably at least 95%, preferably at least 97%, more preferably atleast 99%, and even 100%, of the surface area of the core of thecatalyst grains according to the invention is covered by at least oneexternal layer. This covering imparts an improved diffusionalselectivity to each grain.

According to the invention, the chemical composition of the core can bethe same as or different from that of the external layer. The chemicalcomposition of the core is preferably different from that of theexternal layer. The crystallographic structure of the core can beindependent of that of the external layer, i.e., it can be identical ordifferent.

Each grain of the catalyst according to the invention can have severalexternal layers. According to the invention, at least one of theselayers is uniform with a uniformity in accordance with the criterionpresented above. This layer advantageously coats at least 95%,preferably at least 97%, more preferably at least 99%, and even 100%, ofthe surface area of the core of the grains or the lower layer on whichit is supported.

Each grain of the catalyst according to the invention preferablycomprises a single uniform external layer and covers at least 95% of thesurface of the core of the grains.

The core of a catalyst grain according to the invention can be anyporous structure that has a pore size of between 0.1 nm and 50 nm.

The size of the core of the grains of the catalyst according to theinvention is between 0.1 micron and 0.4 mm. The size of the core of thegrains of the catalyst according to the invention is preferably between0.2 and 100 microns, more preferably between 0.5 and 20 microns.

The core of each of the grains of the catalyst according to theinvention can be a crystallized microporous solid or a structuredmesoporous solid. The core of a grain can comprise a single crystal or anumber of crystals so as to form an agglomerate. In the case of acrystallized microporous solid, the diameter of the micropores of saidsolid can be between 0.1 and 2 nm. In the case of a structuredmesoporous solid, the diameter of the mesopores of said solid can bebetween 2 and 50 nm.

The crystallized microporous solids can be selected from the group thatis formed by the alumino-phosphates, the metallo-alumino-phosphates, thesilicates, the metallo-silicates, in particular the zeolites that aredescribed in the atlas of the zeolites (Atlas of Zeolite FrameworkTypes, Ch. Baerlocher, W. M. Meier, D. H. Olson, Elsevier, 5^(th)Revised Edition, 2001) such as the zeolites that belong to the FAUstructural type (X zeolite, Y zeolite), the BEA structural type (betazeolite), the MFI structural type (ZSM-5 zeolite), the EUO structuraltype (EU-1 zeolite, ZSM-50 zeolite, TPZ-3 zeolite), the NES structuraltype (NU-87 zeolite), the TON structural type (ZSM-22 zeolite, theta-1zeolite, NU-10 zeolite), the MTT structural type (ZSM-23 zeolite), theFER structural type (ferrierite zeolite), the MWW structural type(MCM-22 zeolite), the MEL structural type (ZSM-11 zeolite), the MFSstructural type (ZSM-57 zeolite), the MOR structural type (mordenitezeolite), the MTW structural type (ZSM-12 zeolite), the OFF structuraltype (offretite zeolite), the MAZ structural type (mazzite zeolite), theEMT structural type (EMC-2 zeolite) or the NU-86, NU-88, IM-5, EU-2,ZBM-30, ZSM-48 and IM-12 zeolites.

The structured mesoporous solids can preferably be selected from thegroup that is formed by the MCM-41, MCM-48 and SBA-15 solids.

According to this invention, the core of a grain of the catalyst isacidic. The acidic nature of the core can be measured by any means knownto one skilled in the art, such as, for example, by adsorption ofpyridine or lutidine measured by infrared analysis or bythermogravimetric analysis.

The external layer of each grain of the catalyst of the invention ispreferably a crystallized microporous solid. The crystallizedmicroporous solid of the external layer has pores that have a diameterof preferably between 0.1 and 2 nm, more preferably between 0.1 and 1.5nm, and even more preferably between 0.1 and 1 nm.

The external layer can be selected from the group that is formed by thealumino-phosphates, the metallo-alumino-phosphates, the silicates, themetallo-silicates, in particular the zeolites that are described in theatlas of zeolites (Atlas of Zeolite Framework Types, Ch. Baerlocher, W.M. Meier, D. H. Olson, Elsevier, 5^(th) Revised Edition, 2001), such asthe zeolites that belong to the FAU structural type (X zeolite, Yzeolite), the BEA structural type (beta zeolite), the MFI structuraltype (ZSM-5 zeolite, silicalite-1 zeolite), the EUO structural type(EU-1 zeolite, ZSM-50 zeolite, TPZ-3 zeolite), the NES structural type(NU-87 zeolite), the TON structural type (ZSM-22 zeolite, theta-1zeolite, NU-10 zeolite), the MTT structural type (ZSM-23 zeolite), theFER structural type (ferrierite zeolite), the MWW structural type(MCM-22 zeolite), the MEL structural type (ZSM-11 zeolite, silicalite-2zeolite), the MFS structural type (ZSM-57 zeolite), the MOR structuraltype (mordenite zeolite), the MTW structural type (ZSM-12 zeolite), theOFF structural type (offretite zeolite), the MAZ structural type(mazzite zeolite), the EMT structural type (EMC-2 zeolite), the LTAstructural type (A zeolite) or the NU-86, NU-88, IM-5, EU-2, ZBM-30,ZSM-48 and IM-12 zeolites.

The core and the external layer are preferably zeolites. The zeolite ofthe core differs from the zeolite of the external layer by thestructural type, the chemical composition of the crystalline frameworkand/or by the nature of the compensating cations; very preferably, thezeolite of the core differs from the zeolite of the external layer bythe chemical composition of the crystalline framework.

The combinations of zeolites to form the core-external layer unit can beselected from all the zeolites that are described in the atlas ofzeolites (Atlas of Zeolite Framework Types, Ch. Baerlocher, W. M. Meier,D. H. Olson, Elsevier, 5^(th) Revised Edition, 2001).

The average thickness of the external layer over all of the grains ofthe catalyst can be variable based on the catalysts and also, for adetermined catalyst, based on reactions considered and experimentalconditions, in particular temperature, pressure and/or the rate ofcirculation of the fluid. The average thickness of the external layerover all of the grains is defined by the formula:

$C = {\frac{2}{N}*{\sum\limits_{i = 1}^{N}\;\left( {{{Ei}\;\max} + {{Ei}\;\min}} \right)}}$

The average thickness of the external layer over all of the grains ispreferably between 0.01 and 100 microns, more preferably between 0.1 and10 microns.

The core of a grain, and, because of the uniformity of the externalgrain, the grain itself can have any shape, preferably a spherical,cylindrical or ellipsoidal shape, more preferably a spherical shape.

A grain generally comes in the same form as the core because of theuniformity of the external layer of said grain.

Advantageously, the core of a grain of the catalyst represents at least10% and at most 99% of the total volume of said grain. The radius of thecore of this grain can represent at least 40%, more advantageously atleast 60%, and even more advantageously at least 70% of the total radiusof said grain.

The catalyst according to the invention can contain one or moreelements, in particular metals or their cations, or compounds of theseelements, in particular metal oxides. The catalyst according to theinvention can comprise at least one metal that is selected from thegroup that is formed by the elements Cu, Ag, Ga, Mg, Ca, Sr, Zn, Cd, B,Al, Sn, Pb, V, P, Sb, Cr, Mo, W, Mn, Re, Fe, Co, Ni, Pt, Pd, Re and Rh.

The catalyst according to the invention can comprise one or morehydrogenation/dehydrogenation component(s), such as the metals Ni, Co,Pt, Pd, Re and Rh.

The catalyst according to the invention is generally in cationic form,for example in hydrogen form or in ammonium form.

The catalyst according to the invention can comprise a binder that makesit possible to keep the grains together, optionally in a particularform, for example in the form of a pellet, an extrusion product, ball orpowder. The binder can also have an inert diluent function, for exampleto control the activity per unit of weight of catalyst. Thus, the bindercan comprise one or more cations or oxides derived from elementsselected from the group that is formed by Cu, Ag, Ga, Mg, Ca, Sr, Zn,Cd, B, Al, Sn, Pb, V, P, Sb, Cr, Mo, W, Mn, Re, Fe, Co, Ni, Pt, Pd, Reand Rh.

Another object of this invention relates to a process for preparation ofthe catalyst according to the invention, i.e., a catalyst that comes inthe form of grains, each grain being formed by a core that consists ofan acidic porous solid, whereby said core is covered by a uniformexternal layer. Said process comprises:

-   -   a) A stage for preparation of crystals or crystal agglomerates        to form the core of each of the grains, whereby the core        consists of a porous solid,    -   b) A stage for modification of the core of each grain to impart        an at least partial acidity to said core,    -   c) A stage for adhesion of nuclei distributed uniformly over the        surface of the core of each grain, said nuclei consisting of        nanocrystals based on the material used for the external layer,        and    -   d) A stage of growth of said nuclei on the core of each grain.

The order of presentation of the stages of the process of the inventiondoes not necessarily correspond to the order of carrying out thesestages. By way of example, stage b) can be carried out directly on thecore of a grain after stage a) for preparation of said grain or,alternately, on a composite that comprises the core and its externallayer, after stage d) of the process.

During stage a), the core of each of the grains of the catalystaccording to the invention can be prepared by any means known to oneskilled in the art. The crystals and crystal agglomerates for formingthe core of each of the grains are prepared during a single synthesisstage corresponding to stage a).

During stage b) of the invention, a modification of the core of eachgrain is carried out to impart an at least partial acidity to said core.This modification can be carried out by any means known to one skilledin the art.

For example, in the case of a core with a zeolite base that comprisesions of alkaline metals, modification stage b) should generally make itpossible to eliminate, at least partially, these alkaline metals.

The modification of stage b) imparts to the core an at least partialacidity. This acidity can also be total, i.e., all the exchange sites ofthe core are combined with a proton.

This modification stage b) can be carried out by at least one ionexchange with an acid, in particular, a mineral acid such ashydrochloric acid and/or with an ammonium compound obtained by ionexchange with a solution of an ammonium salt, such as ammonium chloride.The ion exchange can be carried out by means of a thick suspension, onone or more occasions, in an ion exchange solution. The zeolite of thecore is generally calcined before the ion exchange so as to eliminateany absorbed organic substance to the extent that the ion exchange isthereby facilitated. The ion exchange can be carried out by any meansand in any operating condition that is known to one skilled in the art.

Before the implementation of stage c) of the process of the invention,the core optionally can undergo various treatments. For a core that isbased on zeolite or mesoporous material, standard heat and/or chemicalmodification treatments that are known to one skilled in the art can beconsidered, in particular ion exchange operations to put the zeolites inthe desired cationic form.

Surface treatments optionally can be performed to extract the elementsthat are harmful to the implementation of stages c) and d) of theprocess of the invention so as to promote the reactivity of the coreand/or the fixation of nanocrystals from which the external layer willgrow.

During stage c) of the process of the invention, nuclei that aredistributed uniformly adhere to the surface of the core of each grain,whereby said nuclei consist of nanocrystals based on the material thatis used for the external layer.

The nanocrystals may have a size of between 40 and 500 nm, preferablybetween 50 and 400 nm, and more preferably between 60 and 200 nm.

This adhesion can be carried out by any means known to one skilled inthe art. By way of example, this adhesion can be carried out withchemical bonding agents or a grafting agent.

Alternatively, this adhesion can be carried out with agents forreversing the surface charge, such as, for example, the cationicpolymers described by V. Valtchev et al. (“Zeolites and MesoporousMaterials at the Dawn of the 21^(st) Century,” Proceedings of the13^(th) International Zeolite Conference, Montpellier, France, 8-13 Jul.2001, Studies in Surface Science and Catalysis, Vol. 135, p. 298).

This adhesion can be carried out by, for example, mixing the corezeolite with the nuclei of the zeolite of the layer, in a stirred andaqueous medium, after a polymer is adsorbed on one of the two zeolites,which reverses the surface charge and ensures an electrostaticconnection. The adsorption can be carried out in an aqueous stirredmedium with one of the two zeolites, for example with a cationic polymerat a pH of more than 7.

The nanocrystals that constitute the nuclei are generally zeolites thatcan be synthesized by a method for synthesis of colloidal zeolites, forexample by a so-called “clear solution” method as it is described in thearticle “Small Particles Technology,” J. E. Otterstedt, D. A. Brandreth,Plenum Press, 1998.

During stage d) of the invention, the nuclei that have adhered to thecore of each grain undergo growth. The growth of the external layer,which is generally a zeolite, can be carried out in one or moreoperation(s), for example by immersion of the core, on which the nucleihave been deposited, in a stirred and aqueous medium or underhydrothermal conditions.

The formation of the external layer by growth of nuclei distributeduniformly over the surface of the core of each grain involves adiscontinuity between the core and the layer. This discontinuity is atall points of the core/external layer junction for each of the grains ofthe catalyst. This discontinuity is material, i.e., the overallstructure of the catalyst according to the invention is not homogeneousto the extent that it has, for each grain, a core and an external layerthat are identified. This discontinuity between the core and the layercan be observed by electronic microscopy (scanning, transmission?). Thejunction between the core and the layer makes it possible todistinguish, for each grain of the catalyst according to the invention,the presence of a core and that of an external layer that covers saidcore.

In addition to the modification operations carried out during stage b)of the process of the invention, the process can comprise additionalstages for introducing an additional active phase in each grain, andoptionally in the binder, forming the catalyst. These additional stagescan be carried out at any point in the process of the invention. Theymay relate to the core of the grain, to the external layer of thisgrain, to the entirety of the grain, namely the core and the externallayer, or to the binder.

In a general manner, it is possible to replace the cation(s) of crystalsof the core and/or of the external layer of the catalyst by any cationor any metal cation, in particular those of groups IA, IB, IIA, IIB,IIIA, and IIIB, including the rare earths, and those of group VIII,including the noble metals, of the periodic table. It is also possibleto replace this cation or these cations by tin, lead and bismuth. Theexchange is generally carried out with a solution that contains asuitable cation salt, in a way known to one skilled in the art. Theexchange can be done selectively on the core before the deposition ofthe external layer or both on the core and on the external layer.

The process of the invention can comprise a stage for depositing one ormore elements, in particular metals or cations thereof, or compounds ofthese elements, in particular metal oxides. The catalyst according tothe invention can comprise at least one metal that is selected from thegroup that is formed by the elements Cu, Ag, Ga, Mg, Ca, Sr, Zn, Cd, B,Al, Sn, Pb, V, P, Sb, Cr, Mo, W, Mn, Re, Fe, Co, Ni, Pt, Pd, Re and Rh.

This deposition can be carried out at the core and/or at the externallayer surrounding said core. The stage for depositing this or theseelement(s) can be carried out by an ion exchange or by an impregnationwith said element, cation or compound, or with a suitable precursor ofsaid cation or compound. Such an ion exchange or such an impregnationcan be carried out on the zeolite of the core or on the external layer,for example in its crude synthesis form that may or may not be calcined,in hydrogen form and/or in ammonium form and/or in any other exchangedform (metal or not).

In most of the cases of an ion exchange, it is preferable to carry outonly one partial exchange of metal, whereby the remaining sites areoccupied by another cation, in particular the hydrogen or ammoniumcations. In some cases, it may be desirable to introduce two metalcations or more by an ion exchange.

In the cases where the zeolite of the core and/or the layer that encasessaid core are impregnated with a metal compound, the metal compound canbe added with a content of less than 20% by weight, preferably less than10% by weight, and more preferably less than 5% by weight, relative tothe final catalyst weight.

The impregnation and the exchange can be carried out by any means knownby one skilled in the art.

The process of the invention can include an activation treatment. Thesetreatments comprise the reduction, for example in an atmosphere thatcomprises hydrogen, so as to produce a metal or other reduced forms.These treatments can be carried out at any point in the preparation ofthe catalyst. These treatments optionally can be carried outsubsequently during the use of the catalyst inside a reaction zone.

The process of the invention may include a stage for shaping using abinder that makes it possible to keep the grains together. This shapingmay comprise the mixing of the catalyst grains with the binder followedby, for example, extrusion, granulation, drying by atomization or a dropcoagulation of said mixture. The binder optionally can be mixed inadvance with an active compound and can have the function of an inertdiluent to control the activity per unit of weight of catalyst.

The binder can be any substance used in a conventional manner as acatalyst substrate, such as silica, the different forms of alumina,clays such as bentonites, montmorillonites, sepiolite, attapulgite,fuller's earth and synthetic porous materials such as silica-alumina,silica-zirconia, silica-thorin, silica-glucine or silica-titaniumdioxide. Combinations of these binders can be considered within thescope of this invention.

Any suitable method for mixing the grains with a binder, known to oneskilled in the art, can be used, in particular the methods that aresuitable for the shaping of the catalyst in a form of extrudates,pellets, granules, balls or powder.

In the case where the catalyst comprises a metal compound, for example ahydrogenation/dehydrogenation component or another metal that has acatalytic activity, and a binder, the metal compound can be exchanged orimpregnated in the grains or in the mixture with a binder and/or in thegrain-binder composition. In the case where at least a portion of themetal compounds is impregnated or exchanged in the binder, this portionor all of these compounds can be one or more cations or oxides derivedfrom elements that are selected from the group that is formed by Cu, Ag,Ga, Mg, Ca, Sr, Zn, Cd, B, Al, Sn, Pb, V, P, Sb, Cr, Mo, W, Mn, Re, Fe,Co, Ni, Pt, Pd, Re and Rh.

After the formation of the external layer of the grains of the catalyst,heat and chemical modification operations can be conducted to breakdown, for example, the structuring agents or the organic bonding agentsor the organic substrate material if one is used and to put the zeolitesin their desired cationic form.

The catalyst can be shaped by any technique that is known to one skilledin the art, in particular by granulation, by extrusion, by drying, byatomization, or by drop coagulation with a binder. The shaping isadvantageously followed by a drying stage and a calcination stage. Theseshaped solids can also undergo heat and chemical treatments before usein the catalytic processes.

Example 1 Synthesis of a Catalyst that Comprises Grains Consisting of aBeta Zeolite Layer on a Y Zeolite Core

The core is a Y zeolite (FAU structural type) that has undergone amodification treatment imparting to it the desired acidity (USYzeolite). It is obtained from the Zeolyst Company (© CBV780), is definedby an Si/Al ratio of 45, and comes in the form of 4-6 μm agglomeratesthat have 0.4-0.6 μm crystals.

The external layer is a beta zeolite. The formation of the layercomprises the preparation of nuclei, the adhesion of nuclei to the USYzeolite core, and the growth of the nuclei.

The preparation of beta zeolite nuclei that have an Si/Al ratio of about17 is made in the following manner: 0.41 g of aluminum isopropoxide(Aldrich) is hydrolyzed in 6.50 g of 20% TEAOH solution (Fluka,tetraethylammonium hydroxide in aqueous solution, 20% by mass), and asecond solution that contains 6.10 g of freshly freeze-dried colloidalsilica (Akzo Nobel, Bindzil 30/220) dissolved in 20.00 g of 20% TEAOHsolution is prepared. The two solutions are then mixed so as to obtain aclear solution. The final composition of this solution is:9TEAOH:0.25 Al₂O₃:25 SiO₂:295 H₂O

The reaction mixture is introduced into a hermetic polypropylene bottleand put into the oven; the synthesis of the nuclei is done at 80° C. for15 days. Once the synthesis is terminated, the nuclei are washed,dispersed and recovered by successive centrifuging cycles up to a pHthat is close to 7. The pH of the colloidal suspension of the Betazeolite nuclei is then adjusted to 9.5 by adding a 0.1% ammoniasolution. A nanocrystal colloidal suspension that constitutes the nucleiwith an average size of 100 nm results therefrom.

The adhesion is ensured by charge reversal of the USY zeolite crystalagglomerates. A cationic polymer solution (poly(diallyldimethylammoniumchloride, marketed by the Aldrich company) at 0.5% by mass in water isprepared. The pH of this solution to brought to 9.5 by adding a 0.1%ammonia solution.

The USY zeolite and the cationic polymer solution are brought intocontact for 1 hour in a (cationic polymer solution)/(crystals) massratio of 133. The crystals are recovered by decanting. Excess cationicpolymer is eliminated by a washing sequence with an ammonia solution at0.1% by mass in water.

The USY zeolite that is treated by the cationic polymer is brought intocontact for 1 hour with the colloidal suspension of beta zeolite nucleiin a (colloidal suspension)/(crystals) mass ratio of 33. The solid thatconsists of beta zeolite nuclei adhering to the surface of the USYzeolite crystal agglomerates is recovered by decanting. Excess betazeolite nuclei are eliminated by a washing sequence with an ammoniasolution at 0.1% by mass in water.

To eliminate the cationic polymer and to form stable bonds between theUSY zeolite core and the beta zeolite nuclei, the solid that consists ofbeta zeolite nuclei adhering to the surface of the USY zeolite core issubjected to a heat treatment (in air) comprising:

-   -   A rise in temperature from ambient temperature to 200° C. in 10        minutes,    -   A plateau of 1 hour,    -   A rise from 200° C. to 550° C. in 4 hours,    -   And a plateau at 550° C. for 4 hours.    -   The return to ambient temperature is done with the inertia of        the furnace.

The growth of nuclei begins by an immersion of the solid that consistsof beta zeolite nuclei adhering to the surface of the USY zeolitecrystal agglomerates in an amount of synthesis solution defined abovesuch that its mass is 100× greater than that of the crystals then keptat 100° C. for 7 days. The solid composite that consists of a betazeolite layer adhering to the surface of the USY zeolite crystalagglomerates is recovered by decanting and washed with distilled water.The composite is then filtered and dried at 100° C. for 12 hours andcalcined under the conditions of the heat treatment above.

The layer that is formed has an average thickness of 500 nm and auniformity criterion of 0.1.

The composite that consists of a beta zeolite layer adhering to thesurface of the USY zeolite agglomerates is mixed with an SB3-typealumina gel provided by the Sasol Company. The mixed paste is thenextruded through a die with a diameter of 1.4 mm. The extrudates thatare thus obtained are calcined at 500° C. for 2 hours in air. Thecontent by weight of the beta/USY composite is 50% by weight.

Example 2 Synthesis of a Catalyst that Comprises Grains that Consist ofa Silicalite-1 Zeolite Layer on a Beta Zeolite Core

The core is a beta zeolite (BEA structural type) prepared according tothe following method. Metal aluminum is dissolved in a 20% TEAOHsolution (tetraethylammonium hydroxide in aqueous solution, 20% by mass)at ambient temperature, then centrifuged to eliminate the remainingimpurities. The TEOS (tetraethoxysilane) is then added, and the mixtureis left to be stirred until a dense gel is obtained (evaporation ofalcohol and water at ambient temperature to obtain the necessary 6 molof water). The addition of HF brings about the formation of a solid thatit is necessary to break up as finely as possible before introducing itinto the jacket of the autoclave. The synthesis is performed at 140° C.for 9.5 days; the final composition of the gel is:0.55TEAOH:0.02 Al₂O₃:1 SiO₂:0.6 HF:6 H₂O

The reaction mixture is introduced into a Teflon-jacketed autoclave andput into the oven. The water that is present plays the role of solventand ensures an autogenous pressure: the pressure inside the autoclave isequal to the saturating vapor pressure of the water at the temperatureof the synthesis. Once the synthesis is terminated, the crystals arerecovered in a filter, washed and then dried.

The beta zeolite can be defined by an Si/Al molar ratio of 25 and anaverage size of 20 μm.

The beta zeolite crystals are subjected to modification operations,i.e., to heat treatments and ion exchange operations. The first heattreatment in air comprises:

-   -   A rise in temperature from ambient temperature to 200° C. in 10        minutes,    -   A plateau of 1 hour,    -   A rise from 200° C. to 550° C. in 4 hours,    -   And a plateau at 550° C. for 4 hours.    -   The return to ambient temperature is performed with the inertia        of the furnace.

These crystals have a specific surface area of about 600 m²/g.

The ion exchange is operated by suspending the beta zeolite in anammonium nitrate solution with a 10 M concentration at about 100° C. for4 hours. The crystals are recovered by filtration, washed with permutedwater, then dried in an oven at 100° C. for 16 hours, and are subjectedtwo other times to ion exchanges, filtrations, and washing and dryingcycles.

A second heat treatment is then ensured under the same conditions as thefirst.

The external layer is the silicalite-1 zeolite. The formation of thelayer comprises the preparation of the nuclei, their adhesion to thebeta zeolite core, and the growth of the nuclei.

An aqueous suspension of nuclei that consist of nanocrystals that have asize of 100 nm of purely silicic MFI-structural-type zeolite(silicalite-1) is prepared. The zeolite mass in suspension is 4%.

The preparation of the silicalite-1 nanocrystals is performed in thefollowing manner: 40.00 g of TEOS (Fluka, tetraethoxysilane) ishydrolyzed in 70.28 g of 20% TPAOH solution (Fluka, tetrapropylammoniumhydroxide in aqueous solution, 20% by mass); 10.16 g of water is added,and the solution is then stirred so as to obtain a clear solution. Thefinal composition of the solution is:9TPAOH:25 SiO₂:480 H₂O:100 EtOH

The reaction mixture is introduced into a hermetic polypropylene bottleand put into the oven, and the synthesis is performed at 80° C. for 4days. Once the synthesis has ended, the crystals are washed, dispersedand recovered by successive centrifuging cycles until a pH of close to 7is obtained. The pH of the colloidal suspension of silicalite-1nanocrystals is then adjusted to 9.5 by adding a 0.1% ammonia solution.

The adhesion of the silicalite-1 zeolite nuclei is ensured by reversingthe charge of the beta-core zeolite. The beta zeolite crystals (BEAstructural type) are subjected to the treatment described below.

A solution of cationic polymer (poly(diallyldimethylammonium chloride,marketed by the Aldrich Company) at 0.5% by mass in water is prepared.The pH of this solution is brought to 9.5 by adding a 0.1% ammoniasolution.

The beta zeolite crystals and the cationic polymer solution are broughtinto contact for 1 hour in a (cationic polymer solution)/(crystals) massratio of 133. The crystals are recovered by decanting. Excess cationicpolymer is eliminated by a sequence of washing with an ammonia solutionat 0.1% by mass in water.

The beta zeolite crystals that are treated by the cationic polymer arebrought into contact for 1 hour with the colloidal solution in a(colloidal solution)/(crystals) mass ratio of 33. The crystals arerecovered by decanting. Excess MFI-structural-type zeolite is eliminatedby a sequence of washing crystals with an ammonia solution at 0.1% bymass in water.

To eliminate the cationic polymer and to form stable bonds between thebeta zeolite core and the silicalite-1 nuclei, the composite solid issubjected to the heat treatment that is described above.

The growth of the nuclei begins by an immersion of the composite solidthat consists of silicalite-1 zeolite nuclei adhering to the surface ofbeta zeolite crystals in a clear synthesis solution defined by the molarcomposition 3TPAOH:25SiO₂:1500H₂O:100EtOH, such that its mass is 100×greater than that of the crystals but kept at 200° C. for 45 minutes.

This solution is obtained by mixing 28.92 g of TEOS, 136.54 g ofdistilled water and 16.94 g of TPAOH at 20% by mass in water.

The composite that consists of a silicalite-1 zeolite layer adhering tothe surface of the beta zeolite crystals is recovered by decanting andwashed with the distilled water. The composite is then filtered anddried at 100° C. for 12 hours.

The operations of adhesion and growth are repeated a second time with agrowth period of 60 minutes (instead of 45 minutes).

The silicalite-1 layer that is formed has an average thickness of 1100nm and a uniformity criterion of 0.08.

The composite that consists of a silicalite-1 zeolite layer adhering tothe surface of the beta zeolite crystals is mixed with an SB3-typealumina gel provided by the Sasol Company. The mixed paste is thenextruded through a die with a diameter of 1.4 mm. The thus obtainedextrudates are calcined at 500° C. for 2 hours in air. The content byweight of the silicalite-1/beta composite is 50% by weight.

1. A zeolite catalyst consisting essentially of grains, each grain beingformed by a zeolite core covered by at least one external zeolite layerhaving a different crystallographic structure than that of the core, thecore consisting essentially of an acidic crystallized microporous solidzeolite and having a size of between about 0.5 micron and about 20microns, wherein the external layer is a crystallized microporous solidzeolite and has a uniform thickness with a uniformity criterion, C,which is less than 0.30, whereby said uniformity criterion C is definedas being equal to an average, on a number N of catalyst grain samples,of the ratio of the difference between the maximum thickness, Ei_(max),of the external layer and the minimum thickness, Ei_(min), of this samelayer to the average of these two thicknesses Ei_(max), and Ei_(min),and with the provision that the grains have a spherical shape and havean overall average thickness of the external layer of between 0.1 and 10microns yielding a maximum average grain size of about 40 microns.
 2. Acatalyst according to claim 1, wherein at least 95% of the surface ofthe core of the grains is covered by at least one external layer.
 3. Acatalyst according to claim 1, wherein the chemical composition of thecore is different from that of the external layer.
 4. A catalystaccording to claim 1, wherein the crystallized microporous solid of theexternal layer has pores that have a diameter of between 0.1 and 2 nm.5. A catalyst according to claim 1, wherein N is at least
 100. 6. Acatalyst according to claim 1, wherein the uniformity criterion C isless than 0.2.
 7. A catalyst according to claim 1, wherein theuniformity criterion C is less than 0.1.
 8. A catalyst according toclaim 6, wherein N is at least
 100. 9. A catalyst according to claim 7,wherein N is at least
 100. 10. A catalyst according to claim 9, whereinat least 99% of the surface of the core of the grains is covered by atleast one external layer.
 11. A catalyst according to claim 1, whereinthe grains consist essentially of a beta zeolite layer on a Y zeolitecore and the uniformity Criterion is about 0.1.
 12. A catalyst accordingto claim 1, comprising grains consisting essentially of a silica lite-1zeolite layer on a beta zeolite core and wherein the silica lite-1 layerhas an average thickness of about 1100 nm and a uniformity Criterion ofabout 0.08.
 13. A catalyst according to claim 1, wherein thecrystallized microporous solid of the external layer has pores that havea diameter of between 0.1 and 1.5 nm.
 14. A catalyst according to claim1, wherein the crystallized microporous solid of the external layer haspores that have a diameter of between 0.1 and 1 nm.